1. Field of the Invention
This invention relates to a method for the production of silicon carbide single crystals, and more particularly, it relates to a method for the production of silicon carbide single crystals, by which silicon carbide single crystals not only having a high degree of surface flatness but also having stacking faults and antiphase boundaries both greatly reduced can be grown on a silicon single-crystal substrate.
2. Description of the Prior Art
Silicon carbide (SiC) is a semiconductor material having a wide band gap of 2.2 to 3.3 electron-volts (eV), and is thermally, chemically and mechanically stable and also has great resistance to radiation damage. On the other hand, there arise various problems in semiconductor devices made of conventional semiconductor materials such as silicon (Si) when they are driven under difficult conditions such as high temperatures, high output power operation, and exposure to radioactive rays. Thus, SiC can be used as a semiconductor material for semiconductor devices which are required to be driven under such difficult conditions, and accordingly these semiconductor devices can be applied to a wide variety of fields.
Despite these many advantages and capabilities, SiC has not yet been placed in actual use because the technique for growing high-quality SiC single crystals with high productivity on an industrial scale has yet to be developed.
Conventional processes for preparing SiC single crystals on a laboratory scale include growing an SiC single crystal by the use of the so-called sublimation method (i.e., the Lely method); and growing an SiC single-crystal layer by an epitaxial growth method such as chemical vapor deposition (CVD) or liquid phase epitaxy (LPE) on an SiC single-crystal substrate obtained by the Lely method. The size of the SiC single crystal thus produced by each of the conventional processes is sufficient to produce semiconductor device elements thereon.
In such conventional processes, however, the resultant single crystals are still small in size, and it is difficult to control the size and shape of each single crystal with high accuracy. Moreover, it is also difficult to control the polytype and the impurity concentration of these single crystals.
In recent years, the inventors have developed a process for growing a large-sized high-quality SiC single crystal on an inexpensive and readily-available Si single-crystal substrate by the chemical vapor deposition (CVD) technique and filed a Japanese Patent Application No. 58-76842 (76842/1983) which corresponds to U.S. Pat. No. 4,623,425. Also, another process for growing a large-sized SiC single crystal by the carbonization CVD technique is disclosed in Applied Physics Letters, 42(5), Mar. 1, 1983. This process includes heating the surface of an Si single-crystal substrate in an atmosphere containing hydrocarbon gas to form an SiC thin film thereon by carbonization and then growing an SiC single-crystal layer on the SiC thin film by the CVD method. These techniques are referred to as hetero-epitaxial growth methods in association with the growth of single-crystal layers on a single-crystal substrate which is made of a different material from that of the single-crystal layers.
In general, however, when such a hetero-epitaxial growth method is used to form an epitaxially grown layer on a single-crystal substrate, the epitaxially grown layer has a tendency to contain crystal defects, inter alia, stacking faults, because there is a difference in lattice constant, coefficient of thermal expansion, and chemical bonding between the epitaxially grown layer and the single-crystal substrate.
For example, the lattice constant of Si single crystals is different from that of SiC single crystals by as much as about 20%, so that many stacking faults arise on the {111} planes in the SiC single-crystal layer grown on the Si single-crystal substrate. Such stacking faults develop on the faces of a regular octahedron which has its one apex on the interface between the Si single-crystal substrate and the grown layer of the SiC single crystal. On the surface of the grown layer of the SiC single crystal, there appear some defect patterns, each of which has a shape corresponding to a certain section of the regular octahedron. For example, when an Si (100) substrate is used, these defect patterns take the shape of squares with their sides all parallel to the &lt;011&gt; direction. These stacking faults exert an adverse effect on the electronic properties of the resultant SiC single crystals. Thus, semiconductor devices with excellent device characteristics cannot be produced by using such SiC single crystals.
Furthermore, when an SiC single-crystal layer is grown on an Si (100) substrate, the resultant SiC single crystal contains crystal defects referred to as antiphase boundaries, so that semiconductor device elements cannot be formed at desired positions on the resultant SiC single-crystal substrate.
Therefore, none of the growth methods set forth above can provide SiC single crystals having these crystal defects greatly reduced, and maintain high reproducibility.
Recently, another method of growing SiC single crystals has been reported, in which an SiC single crystal is grown on an Si single-crystal substrate having a growth plane with a crystal orientation inclined from the [100] direction toward the &lt;011&gt; direction (e.g., toward the [011] direction or the [011] direction) (K. Shibahara, et al., Appl. Phys. Lett., 50(1987) 1888; and H. S. Kong et al., J. Mater. Res. 3(3), May/June 1988). In this method, antiphase boundaries can be removed, but the density of stacking faults cannot be reduced. Thus, there is a need for a method of producing SiC single crystals having stacking faults and antiphase boundaries both greatly reduced, with stable productivity on an industrial scale.